Salt dome drilling method

ABSTRACT

A method of drilling or testing for potential hydrocarbonproducing zones adjacent a salt dome, shale dome, fault plane, or other geologic surface or features including the steps of drilling vertically, deviating the direction of drilling to a slant from vertical, drilling at said slant to a predetermined position above the geological surface where the hole is returned to vertical (or near vertical), and then deviated to a slant approximately paralleling the geologic surface in order to penetrate a maximum number of potential objectives at optimum positions.

United States Patent [72] inventor DonaldLPhu MetalrieJA. [21] Appl. No. 8,225 [22] Filed Feb.3, 1970 [45] Patented Ang.10,1971 [731 Assignee SbelOilConpuy New York, NJ.

[54] SALTDOME DRILLINGMETHOD 6 Chill, 6 Drawing Figs.

[52] 0.8.61. 175161 [51] E21b7/06 [50] Flelrlolsearch l75/6l,57,

[56] References Cited UNITED STATES PATENTS 2,179,832 11/1939 Smith 166/255 X 2,565,794 8/1951 Young...... 175/61 3,042,125 7/1962 Duncan.... 175/61 X 3,412,815 11/1968 Holseretal... 175/61X 3,518,840 7/1970 Mertz 175/61 X OTHER REFERENCES Close, More Oil From Crooked Wells," Scientific American, August 1939, pp. 84- 87, 175/45 Gatlin, Petroleum Engineering, Drilling And Well Comple, tions, (1960) Prentice-Hall, lnc., Englewood Cliffs, N..l., pp. 156 I61, 350

Primary Examiner-Stephen J. Novosad Attorneys-J. H. McCarthy and Theodore E. Bieber ABSTRACT: A method of drilling or testing for potential hydrocarbon-producing zones adjacent a salt dome, shale dome, fault plane, or other geologic surface or features including the steps of drilling vertically, deviating the direction of drilling to a slant from vertical, drilling at said slant to a predetermined position above the geological surface where the hole is returned to vertical (or near vertical), and then deviated to a slant approximately paralleling the geologic surface in order to penetrate a maximum number of potential objectives at optimum positions.

PATENTEDAUBIOIHYI 3.598.190

sum 1 or 3 FIG. INVENTOR:

DONALD J. PFAU HIS ATTORNEY PATENTED AUG 1 0 I97! SHEET 3 [IF 3 FIG. 3

INVENTOR DONALD J- PFAU BY:

HIS ATTORNEY SALT DOME DRILLING METHOD This invention relates to a method of directionally drilling or surveying adjacent to a geologic feature, such as a salt dome and more particularly to a method of exploring the formations immediately surrounding the geologic feature in an attempt to locate hydrocarbon-producing zones. Drilling adjacent to a salt dome is used herein as an example, although the method has equal utility in drilling associated with other geologic features. Whenever a salt dome structure is located which includes potential hydrocarbon reservoir formations spaced around the various domal configurations, the problem presents itself as to how to drill into the potential hydrocarbon-bearing formations to establish production in the most efficient manner.

The customary procedure for accomplishing this on land has been to position the drilling rig over a selected position along the dome in as accurate a manner as possible. Drilling is then commenced downwardly to one side of the dome at spaced circumferential intervals so that the formations around the domal mass are effectively tested for hydrocarbons. After a well is completed with respect to one area along the salt dome, the drilling rig may be readily moved and positioned over another point along the periphery so that the drilling operations may be repeated with respect thereto. On land, a drilling rig may be moved several times to drill on the several sides of a salt dome.

In an attempt to locate new oil and/or gas fields, however, an ever-increasing amount of well drilling has been conducted at offshore locations, such for example, as off the coast of Alaska, California, Louisiana and Texas. Although mobile and floating drilling rigs are often used forexploratory drilling purposes in such offshore locations, development drilling of a large number of wells in certain water depths can be carried out most readily through the use of fixed platforms which are erected on piles driven into the seabed. These fixed drilling platforms are not only of sturdy construction permitting them to withstand heavy seas, but are particularly suitable for drilling from a single platform a cluster of as many as 24-40 or more directionally deviated development wells.

In the event, however, that the potential oil or gas field includes structural traps associated with a salt dome, the use of fixed drilling platforms for development drilling presents certain difficulties since it would not be economically feasible or practical to position such a platform over each and every position along the dome where a well is to be drilled. In the past, it has been attempted to solve this problem by selecting predetermined locations adjacent a salt dome and slant drilling directly to these locations from the fixed platforms.

This approach, however, has decided disadvantages. Due to the combined geometry of the domal surface, the associated potential hydrocarbon reservoirs, and the course of the wells, any given well may penetrate only one, or else very few, of the objective geologic beds at favorable structural positions. The selected spacing of the predetermined drilling locations when using the above-described prior approach may result in poten tial hydrocarbon accumulations being missed if such locations are spread too far apart for economy reasons.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide an improved method whereby all or a greater portion of a geological feature such as a salt dome may be efficiently explored and/or developed as to associated hydrocarbon accumulations from a fixed offshore drilling platform. However, the invention can apply equally well to other offshore and onshore drilling where factors restrict the selection of rig locatrons.

It is a further object of the present invention to provide a method whereby one fixed drilling platform may be utilized to explore for potential hydrocarbon-producing areas on more than one geologic feature.

These and other objectives have been attained in the present invention by providing a method of drilling for potential hydrocarbon-producing zones at various positions along a geologic feature (such as a salt dome) from a drilling platform which is positioned near the feature. The method includes the steps of drilling to a slant from vertical, drilling at said slant to a predetermined position or alignment target, above the selected domal mass, and returning the well to (or near) vertical. At the appropriate depth, drilling is then once again deviated to a. slant approximately paralleling the surface of the feature. In this manner, the drilling rig has been in effect, repositioned over the feature at the location of the alignment target, whereby drilling may be oriented down the desired surface of he feature in an attempt to locate hydrocarbonproducing zones. Several positions along the periphery of a dome, or several features surrounding the rig may be surveyed in this manner consecutively or at the same time. Each well thus drilled may accomplish what would require several wells utilizing common directional drilling practices.

The above-noted objects as well as other objects of this invention will be understood from the following detailed description taken with reference to the drawings wherein:

FIG. 1 is a diagrammatic section view illustrating an offshore fixed drilling platform from which testing and development of the potential hydrocarbon-producing zones surrounding a salt dome may be conducted according to the method of the present invention;

FIG. 2a is a diagrammatic section view illustrating the results of a prior art method of exploring the area surrounding a salt dome from a fixed platform;

FIG. 2b is a diagrammatic plan view showing how present art methods utilizing a platform rig might be used to develop a number of hydrocarbon reservoirs adjacent to a salt dome;

FIG. 20 is a diagrammatic plan view showing the most efficient method for developing the same reservoirs if no restrictions are imposed on rig location;

FIG. 2d is a diagrammatic plan view showing how the subject invention can be used to effectively accomplish the most efficient method of development from a fixed platform; and

FIG. 3 is a diagrammatic view illustrating the manner in which the method according to the present invention may be utilized in surveying more than one geologic feature in a field from a single platform.

Referring to FIG. 1 of the drawing, a drilling platform 11 is illustrated as being positioned on a seabed l2, underlying a body of water 13, by means of a plurality of platform columns such as columns 14 and 15. Each of the platform columns are affixed to foundation piles or casing members such as casing members 16 and 17 by any acceptable means. The casing members or piles upon which the platform is positioned will have been driven previously into the seabed and may be additionally secured therein, as by means of cement (not shown). Platform 11 has associated therewith conventional welldrilling equipment, such as derrick 18, needed during welldrilling operations. For purposes of clarity the derrick 18 is illustrated as being positioned over a drilling slot or well 19 which extends through the platform 11. In actual practice, many development drilling platforms of the fixed offshore type are adapted to permit the drilling of a plurality of directionally deviated development wells, often with a number of such wells being drilled through the various upstanding platform columns.

Disposed beneath the seabed 12 and either beneath or laterally displaced from platform 11 is a typical geologic feature wherein the teachings of the present invention may be carried out. The illustrated feature is a salt dome 20 which has thrust itself under great pressure into and/or through the overlying earth strata as shown although it should be understood that the teachings of the present invention have equal application to other geologic features, such as shale domes or fault planes for example. It should be understood that the dome may be laterally displaced from the platform by many hundreds or even thousands of feet and may be a distance of like magnitude under seabed 12. In many cases of domes of this type, conditions highly favoring the concentration of gas and oil are often found. Porous formations, such as sand formations 21, have been truncated and effectively sealed by the dome creating structural oil traps such as traps 22a22e, inclusive. In this manner trapped oil accumulates against the salt plug in the manner illustrated. Oil or gas accumulations also often occur above the salt dome in reservoir formations that have been folded by the rising plug, or in the porous remnant of older strata pushed up on top of the salt in the course of its upward movement. For example, a cap rock 24 encompassing reservoir formations (not shown) may be positioned over the salt dome 20.

Referring now to FIG. 2a, this diagrammatic illustration shows a prior art method of testing a given portion of the flank of a salt dome or other similar geologic features for potential hydrocarbon-producing zones or entrapments from a fixed offshore platform which is necessarily positioned at one point with respect to the dome. It is, of course, assumed that the dome or domal mass has been located previously by known geological or geophysical techniques and that some approximation has been made as to the mass of the salt dome and its distance below the seabed 12. Using these factors as a guide, the operator selects certain predetermined target locations into which wells would be drilled. At the same time, however, the number of target locations must be somewhat restricted due to time and cost considerations.

The well is then spudded in and vertical operations with drill string 26 are commenced for a number of feet as predetermined by the operator. The vertical well is then deflected toward a target or series of targets through which the well passes as either a slant hole, or else the hole may be straightened to pass vertically through the target (s). The course of the drilling string may be deflected through the formation by the use of any of the well-known devices for this purpose, such as jetting, whipstocks or knuckle joints, and will be periodically checked by one of several accepted means as the well is being drilled.

After the drill bit has reached the domal material, thereby signifying that no further objectives remain with respect to the first selected location, the well is completed or abandoned as warranted. Assuming that a drilling slot is available from the platform, another well may be drilled at a different angle in an attempt to locate another target location of trapped oil potential. The above-described operations are repeated with respect to each and every target area selected by the operator. FIG. 2a illustrates this procedure in diagrammatic form with target areas A, B, C, D and E being shown.

Drilling from the platform directly toward any given target along the dome may test none or at best one or two, of the potentially hydrocarbon-bearing zones at a structural position favorable for accumulation. A series of such wells may be required to test or develop all of the objectives on a relatively small portion of the domal flank, which may not be economically feasible or may still result in missing some accumulations. FIG. 2b shows in plan view six hydrocarbon reservoir 51, 52, 53, 51, 52' and 53, adjacent to a salt dome. The ascending numbers indicate increasing depth to the reservoirs. Using existing art methods to develop these reservoirs from one platform 54 would require six wells as shown by the broken lines. If no restrictions on rig location are imposed, the most efficient way to develop the reservoir is with two wells from two locations 55 and 56 drilled parallel to the domal surface such that each penetrates three of the accumulations as shown in FIG. 20. The invention provides a method to effectively accomplish this by drilling wells from one platform 54 as described in he following sections as depicted on FIG. 2d.

Returning once again to FIG. 1, a diagrammatic view is presented which illustrates the method of surveying a salt dome according to he present invention. As with the prior art method, the well is spudded in and vertical drilling with drill string 26 is carried out for a distance predetermined by the drilling operator. However, use of a slanted drilling rig to avoid this step is not precluded. Slant drilling operations are then initiated using one of the conventional slant drilling techniques. In this instance, however, the first intended deviated course of the well is such as to aim for a target location at a selected position above the salt dome, usually where the rig would preferably be located for the well if it were not confined to the platform location. This first target may not be in proximity to any potential hydrocarbon accumulation, but is specifically designed to allow the remainder of the course of the well to be drilled in optimum alignment with a series of targets on objective formations. It is, of course, assumed that geological and/0r geophysical surveys have been made previously so that the approximate location of the subject salt dome and the depth thereof has been determined. The alignment target location has been indicated generally by reference letter P in FIG. 1. The course of the drilling string is again changed to be vertical at target P in the manner previously described, and as illustrated. Vertical drilling continues to a predetermined depth R where the well is once more deflected to a course approximately parallel to the domal surface in order to penetrate targets on the objective zones. In the event that information on the position of the dome is not sufficiently precise, it may be desirable to penetrate the dome to establish control, and then plug back and sidetrack the well from a depth thereby determined for R. This process can be repeated for as many wells as can be accommodated from the platform or as are economically feasible, within the mechanical limits of the equipment available.

It should be readily understood that the above technique of drilling vertically, slant drilling, and returning to the vertical, effectively places the drilling platform over the salt dome at the position of the second portion of vertical hole, regardless of the lateral distance removed that the platform is actually positioned. From point R the area immediately surrounding a portion of salt dome can be surveyed by merely deviating the drill string at a predetermined angle, and thence drilling downwardly along the side of the dome as illustrated in FIG. 1. For example, the initial development or exploratory well 23 may be drilled as illustrated in solid lines down the left side of the dome as viewed in the figure. It is to be pointed out at this juncture that no attempt has been made to draw the various figures in the drawing to scale. Instead, somewhat stylized presentations have been made to illustrate as clearly as possible the method according to the present invention.

After the first well 23 has been drilled down the side of the salt dome (as seen in FIG. 1) the operator may then elect to drill other such wells as various positions on the dome. He may choose to drill these other wells at any point along the periphery of the salt dome within reach of the platform, since the vertical position of the drill string before the final deflection permits the operator to extend a well in any direction along the slope or side of the dome. in essence, deviation drilling techniques when utilized with the method according to the present invention allow the operator 360 of latitude about which he may drill the lower portion of the hole.

This concept may be seen somewhat more clearly with reference to FIG. 3 wherein a diagrammatic plan view is presented illustrating platform 11 and a portion of a salt dome 20. In accordance with the method of the present invention, a series of wells 26 is shown extending from the platform to various positions on the dome. From the platform, a variety of drilling paths along the slanted face of the dome are possible. The number of separate drilling operations and directions thereof along the dome slopes are determined by time and cost considerations as well as the prospects for finding oil or gas reserves as based on prior drilling along previously selected paths about the slope. It should be understood, of course, that when using a platform having simultaneous multiwell drilling capabilities, two or more locations on the dome may be simultaneously surveyed. In addition, more than one platform may be utilized at any given time with respect to one dome.

With respect to FIG. 3, it may be seen that platform 11 maybe used to survey more than one geologic feature using the technique outline above. In the situation illustrated, in addition to salt dome 20, a fault plane 28 lies under the seabed and ofi'set with respect to platform 11. As illustrated by the broken lines extending from platform 11 to fault plane 28, wells 26 may be drilled vertically, slanted off at the predetermined angle, and again drilled vertically to position the drill bitabove or below this additional geologic feature in the manner previously described with respect to salt dome 20. The bit would then be angled off and drilling along the surface of the plane would begin. it is to be readily understood that more than two geologic features in a field could be surveyed using this approach if conditions so warrant.

As stated above, the method according to the present invention may be utilized not only to explore or develop salt domes but also dome-shaped masses, uplifts, faults, etc. of other composition about which hydrocarbon-producing zones may form, examples of which are volcanic necks and buried coral reefs. It should be also readily apparent that the subject method may be used in the actual drilling of a production well rather than for exploratory purposes. That is, the steps of drilling vertically, drilling at an angle to the vertical, and resuming vertical drilling and then slant drilling may be carried out as above described. Once, however, a producing area is located, the well may be cased substantially along its length with production casing, and a producing wellhead assembly can be affixed thereto in the usual-manner for producing operations, if so desired.

1 claim as my invention:

1. A method of drilling wells along a sloping subsurface geological feature from a single location fixedly positioned with respect to the geologic feature, said method comprising:

drilling a substantially vertical hole in the ground with a drill string and bit to a predetermined depth;

deviating said drilling string and bit at the bottom of said vertical hole and drilling with said drill string and bit at an angle from the vertical to a selected target location over the geologic feature;

resuming substantially vertical drilling with the drill string and bit in a downwardly direction toward said feature; and slant drilling with said string and bit along the feature.

2. The method of claim 1 including the steps of: continuing the resumed vertical drilling until the top of the geologic feature has been located, as necessary to establish control on the feature;

plugging back and sidetracking the hole at a predetermined position above the top of the geologic feature; and

further drilling from the predetermined position downwardly along a first predetermined course along the surface of the feature.

3. The method of claim 2 including the steps of:

completing drilling along said first predetermined course,

and again plugging back the hole so that the bit is located at said predetermined position; and drilling from said predetermined position along another predetermined course along said geologic feature.

4. The method of claim 1 including the additional steps of selectively installing and securing casing and liner at predetermined locations in said well after said vertical and slant drilling operations.

5. The method of claim 4 including the additional step of installing production equipment and wellhead equipment after selective installation of said casing and liner.

6. A method of drilling a well from a fixed drilling location to at least one hydrocarbon-producing zone lying adjacent a geologic feature which is offset laterally with respect to the rig location, said method comprising:

lowering a drill string and bit from said location and drilling a substantially vertical hole in the center underlying the lqcation; retrieving said dl'lll string and bit from said substantially vertical hole after said substantially vertical hole has been drilled to a predetermined depth;

inserting surface casing within said hole;

lowering the drill string and bit into the casing to the bottom of said substantially vertical hole;

drilling with said drill string an bit at an angle from the vertical to a selected target location above the geologic feature;

resuming drilling with the drill string and bit until the hydrocarbon-producing zone is reached with at least the initial portion of the resumed drilling being in a substantially vertical direction;

casing the well with production casing; and

applying a production wellhead assembly to said well whereby said hydrocarbon-producing zone may be produced. 

2. The method of claim 1 including the steps of: continuing the resumed vertical drilling until the top of the geologic feature has been located, as necessary to establish control on the feature; plugging back and sidetracking the hole at a predetermined position above the top of the geologic feature; and further drilling from the predetermined position downwardly along a first predetermined course along the surface of the feature.
 3. The method of claim 2 including the steps of: completing drilling along said first predetermined course, and again plugging back the hole so that the bit is located at said predetermined position; and drilling from said predetermined position along another predetermined course along said geologic feature.
 4. The method of claim 1 including the additional steps of selectively installing and securing casing and liner at predetermined locations in said well after said vertical and slant drilling operations.
 5. The method of claim 4 including the additional step of installing production equipment and wellhead equipment after selective installation of said casing and liner.
 6. A method of drilling a well from a fixed drilling location to at least one hydrocarbon-producing zone lying adjacent a geologic feature which is offset laterally with respect to the rig location, said method comprising: lowering a drill string and bit from said location and drilling a substantially vertical hole in the center underlying the location; retrieving said drill string and bit from said substantially vertical hole after said substantially vertical hole has been drilled to a predetermined depth; inserting surface casing within said hole; lowering the drill string and bit into the casing to the bottom of said substantially vertical hole; drilling with said drill string and bit at an angle from the vertical to a selected target location above the geologic feature; resuming drilling with the drill string and bit until the hydrocarbon-producing zone is reached with at least the initial portion of the resumed drilling being in a substantially vertical direction; casing the well with production casing; and applying a production wellhead assembly to said well whereby said hydrocarbon-producing zone may be produced. 